Is there a way in NHibernate to start with an unproxied model
var m = new Model() { ID = 1 };
m.Name = "test";
//Model also has .LastName and .Age
Now save this model only updating Name without first selecting the model from the session?
If model has other properties then name, you need to initialize these with the original value in the database, unless they will be set to null.
You can use HQL update operations; I never tried it myself.
You could also use a native SQL statement. ("Update model set name ...").
Usually, this optimization is not needed. There are really rare cases where you need to avoid selecting the data, so writing this SQL statements are just a waste of time. You are using an ORM, this means: write your software object oriented! Unless you won't get much advantages from it.
What Stefan says looks like what you need. Please be aware that this is really an edge case and you should be happy with fully loading your entity unless you have some ultra-high-performance issues.
If you simply don't want to hit the database - try using caching - entity cache is very simple and efficient.
If your entity is a huge one - i.e. it contains a blob or something - think about splitting it in two (with many-to-one so that you can utilize lazy loading).
http://www.hibernate.org/hib_docs/nhibernate/html/mapping.html
dynamic-update (optional, defaults to
false): Specifies that UPDATE SQL
should be generated at runtime and
contain only those columns whose
values have changed.
Place dynamic-update on the class in the HBM.
var m = new Model() { ID = 1 };
m = session.Update(m); //attach m to the session.
m.Name = "test";
session.Save(m);
Related
I am trying get a record updated from database with QueryOver.
My code initially creates an entity and saves in database, then the same record is updated on database externally( from other program, manually or the same program running in other machine), and when I call queryOver filtering by the field changed, the query gets the record but without latest changes.
This is my code:
//create the entity and save in database
MyEntity myEntity = CreateDummyEntity();
myEntity.Name = "new_name";
MyService.SaveEntity(myEntity);
// now the entity is updated externally changing the name property with the
// "modified_name" value (for example manually in TOAD, SQL Server,etc..)
//get the entity with QueryOver
var result = NhibernateHelper.Session
.QueryOver<MyEntity>()
.Where(param => param.Name == "modified_name")
.List<T>();
The previous statement gets a collection with only one record(good), BUT with the name property established with the old value instead of "modified_name".
How I can fix this behaviour? First Level cache is disturbing me? The same problem occurs with
CreateCriteria<T>();
The session in my NhibernateHelper is not being closed in any moment due application framework requirements, only are created transactions for each commit associated to a session.Save().
If I open a new session to execute the query evidently I get the latest changes from database, but this approach is not allowed by design requirement.
Also I have checked in the NHibernate SQL output that a select with a WHERE clause is being executed (therefore Nhibernate hits the database) but don´t updates the returned object!!!!
UPDATE
Here's the code in SaveEntity after to call session.Save: A call to Commit method is done
public virtual void Commit()
{
try
{
this.session.Flush();
this.transaction.Commit();
}
catch
{
this.transaction.Rollback();
throw;
}
finally
{
this.transaction = this.session.BeginTransaction();
}
}
The SQL generated by NHibernate for SaveEntity:
NHibernate: INSERT INTO MYCOMPANY.MYENTITY (NAME) VALUES (:p0);:p0 = 'new_name'.
The SQL generated by NHibernate for QueryOver:
NHibernate: SELECT this_.NAME as NAME26_0_
FROM MYCOMPANY.MYENTITY this_
WHERE this_.NAME = :p0;:p0 = 'modified_name' [Type: String (0)].
Queries has been modified due to company confidential policies.
Help very appreciated.
As far as I know, you have several options :
have your Session as a IStatelessSession, by calling sessionFactory.OpenStatelesSession() instead of sessionFactory.OpenSession()
perform Session.Evict(myEntity) after persisting an entity in DB
perform Session.Clear() before your QueryOver
set the CacheMode of your Session to Ignore, Put or Refresh before your QueryOver (never tested that)
I guess the choice will depend on the usage you have of your long running sessions ( which, IMHO, seem to bring more problems than solutions )
Calling session.Save(myEntity) does not cause the changes to be persisted to the DB immediately*. These changes are persisted when session.Flush() is called either by the framework itself or by yourself. More information about flushing and when it is invoked can be found on this question and the nhibernate documentation about flushing.
Also performing a query will not cause the first level cache to be hit. This is because the first level cache only works with Get and Load, i.e. session.Get<MyEntity>(1) would hit the first level cache if MyEntity with an id of 1 had already been previously loaded, whereas session.QueryOver<MyEntity>().Where(x => x.id == 1) would not.
Further information about NHibernate's caching functionality can be found in this post by Ayende Rahien.
In summary you have two options:
Use a transaction within the SaveEntity method, i.e.
using (var transaction = Helper.Session.BeginTransaction())
{
Helper.Session.Save(myEntity);
transaction.Commit();
}
Call session.Flush() within the SaveEntity method, i.e.
Helper.Session.Save(myEntity);
Helper.Session.Flush();
The first option is the best in pretty much all scenarios.
*The only exception I know to this rule is when using Identity as the id generator type.
try changing your last query to:
var result = NhibernateHelper.Session
.QueryOver<MyEntity>()
.CacheMode(CacheMode.Refresh)
.Where(param => param.Name == "modified_name")
if that still doesn't work, try add this after the query:
NhibernateHelper.Session.Refresh(result);
After search and search and think and think.... I´ve found the solution.
The fix: It consist in open a new session, call QueryOver<T>() in this session and the data is succesfully refreshed. If you get child collections not initialized you can call HibernateUtil.Initialize(entity) or sets lazy="false" in your mappings. Take special care about lazy="false" in large collections, because you can get a poor performance. To fix this problem(performance problem loading large collections), set lazy="true" in your collection mappings and call the mentioned method HibernateUtil.Initialize(entity) of the affected collection to get child records from database; for example, you can get all records from a table, and if you need access to all child records of a specific entity, call HibernateUtil.Initialize(collection) only for the interested objects.
Note: as #martin ernst says, the update problem can be a bug in hibernate and my solution is only a temporal fix, and must be solved in hibernate.
People here do not want to call Session.Clear() since it is too strong.
On the other hand, Session.Evict() may seem un-applicable when the objects are not known beforehand.
Actually it is still usable.
You need to first retrieve the cached objects using the query, then call Evict() on them. And then again retrieve fresh objects calling the same query again.
This approach is slightly inefficient in case the object was not cached to begin with - since then there would be actually two "fresh" queries - but there seems to be not much to do about that shortcoming...
By the way, Evict() accepts null argument too without exceptions - this is useful in case the queried object is actually not present in the DB.
var cachedObjects = NhibernateHelper.Session
.QueryOver<MyEntity>()
.Where(param => param.Name == "modified_name")
.List<T>();
foreach (var obj in cachedObjects)
NhibernateHelper.Session.Evict(obj);
var freshObjects = NhibernateHelper.Session
.QueryOver<MyEntity>()
.Where(param => param.Name == "modified_name")
.List<T>()
I'm getting something very similar, and have tried debugging NHibernate.
In my scenario, the session creates an object with a couple children in a related collection (cascade:all), and then calls ISession.Flush().
The records are written into the DB, and the session needs to continue without closing. Meanwhile, another two child records are written into the DB and committed.
Once the original session then attempts to re-load the graph using QueryOver with JoinAlias, the SQL statement generated looks perfectly fine, and the rows are being returned correctly, however the collection that should receive these new children is found to have already been initialized within the session (as it should be), and based on that NH decides for some reason to completely ignore the respective rows.
I think NH makes an invalid assumption here that if the collection is already marked "Initialized" it does not need to be re-loaded from the query.
It would be great if someone more familiar with NHibernate internals could chime in on this.
How would I use nHibernate,configured by fluent nhibernate if it makes any difference, to load an entity using natural/alternate key in some cases, rather than the primary key when using the Load method on an ISession.
I still need the functionality to allow me to do both, and in the majority of cases, the entity will be loaded via the PKey, but in some cases (where an external system is involved), I need to select the record using the natural key.
I'd like to keep the performance benefit Load allows, rather than do a query etc.
// Current
int countryID = 1; // from normal input source
Address a = new Address();
a.Country = session.Load<Country>(countryID);
session.SaveOrUpdate(a);
// Required
string countryCode = "usa"; // from external input source
Address a2 = new Address();
a2.Country = session.LoadViaNatualKeySomehow<Country>(c=> c.Code, countryCode); // :)
session.SaveOrUpdate(a2);
AFAIK, it is not possible. As you can see in Ayendes post, there is a query syntax for criteria, the only natural ID in the whole NHibernate API as far as I know. This query translates into a "normal" query, except of the second level cache handling as described in this post.
It would be nice if it wouldn't at least flush the session.
one simple performance enhancement you can do is turning off auto flush before querying by the (immutable!) natural ID:
session.FlushMode = FlushMode.Never;
session.CreateQuery(...by natural id ...);
session.FlushMode = FlushMode.Auto;
This can make a big difference, but does of course not compete to Load.
The reason why it doesn't exist is most probably the fact the entities in the session are all identified by the id.
If you had it:
var entity1 = session.Load<Entit>(id);
// does not exist
var entity2 = session.LoadByNaturalKey(natural id);
How could NH determine that the id and the natural id are identifying the same object, without loading them from the database? The whole session cache gets into trouble.
how to display the some specific columns of table instead of whole table in entity framework.
using (DataEntities cxt = new DataEntities())
{
notes note = cxt.notes.Where(no => no.id == accID).SingleOrDefault();
return notes;
}
For this purpose, I would suggest you to make use of ViewModel like following :-
notes note = cxt.notes.SingleOrDefault(no => no.id == accID);
var model = new YourViewModel // Your viewModel class
{
ID = note.ID,
PropertyOne = note.PropertyOne, // your ViewModel Property
PropertyTwo = note.PropertyTwo
};
You can do this with QueryView.
This implies editing your model directly in XML as there is no designer support for this, but you will get an independant entity with less fields than the original one.
Advantages:
You can then query data base for this truncated entity directly (you will get only fields you need from the data base - no need to get whole entity from DB and trancate it in code)
It is good in scenarios where you want to send this truncated entity to the
client
with WCF to minimize traffic (e.g. when building big lists on client
that basically need only name and ID and no other entity specific
information).
Disadvantages:
This QueryView-based entity is read only. To make it writeable you will have to add r/w functionality yourself
Producing software for customers, mostly using MS SQL but some Oracle, a decision was made to plunge into Nhibernate (and C#).
The task is to delete efficiently e.g. 10 000 rows from 100 000 and still stay sticked to ORM.
I've tried named queries - link already,
IQuery sql = s.GetNamedQuery("native-delete-car").SetString(0, "Kirsten");
sql.ExecuteUpdate();
but the best I have ever found seems to be:
using (ITransaction tx = _session.BeginTransaction())
{
try
{
string cmd = "delete from Customer where Id < GetSomeId()";
var count = _session.CreateSQLQuery(cmd).ExecuteUpdate();
...
Since it may not get into dB to get all complete rows before deleting them.
My questions are:
If there is a better way for this kind of delete.
If there is a possibility to get the Where condition for Delete like this:
Having a select statement (using LinQ to NHibernate) => which will generate appropriate SQL for DB => we get that Where condition and use it for Delete.
If there is a better way for this kind of delete.
Yes, you could use HQL instead of SQL.
If there is a possibility to get the Where condition for Delete [using Expressions]:
No, AFAIK that's not implemented. Since NHibernate is an open source project, I encourage you to find out if anyone has proposed this, and/or discuss it on the mailing list.
Thanks for your quick reply. Now I've probably got the difference.
session.CreateSQLQuery(cmd).ExecuteUpdate();
must have cmd with Delete From DbTable. On the contrary the HQL way
session.CreateQuery(cmd).ExecuteUpdate();
needs cmd with Delete From MappedCollectionOfObjects.
In that case it possibly solves my other question as well.
There now is a better way with NHibernate 5.0:
var biggestId = GetSomeId();
session.Query<Customer>()
.Where(c => c.Id < biggestId)
.Delete();
Documentation:
//
// Summary:
// Delete all entities selected by the specified query. The delete operation is
// performed in the database without reading the entities out of it.
//
// Parameters:
// source:
// The query matching the entities to delete.
//
// Type parameters:
// TSource:
// The type of the elements of source.
//
// Returns:
// The number of deleted entities.
public static int Delete<TSource>(this IQueryable<TSource> source);
Starting with a List of entities and needing all dependent entities through an association, is there a way to use the corresponding navigation-propertiy to load all child-entities with one db-round-trip? Ie. generate a single WHERE fkId IN (...) statement via navigation property?
More details
I've found these ways to load the children:
Keep the set of parent-entities as IQueriable<T>
Not good since the db will have to find the main set every time and join to get the requested data.
Put the parent-objects into an array or list, then get related data through navigation properties.
var children = parentArray.Select(p => p.Children).Distinct()
This is slow since it will generate a select for every main-entity.
Creates duplicate objects since each set of children is created independetly.
Put the foreign keys from the main entities into an array then filter the entire dependent-ObjectSet
var foreignKeyIds = parentArray.Select(p => p.Id).ToArray();
var children = Children.Where(d => foreignKeyIds.Contains(d.Id))
Linq then generates the desired "WHERE foreignKeyId IN (...)"-clause.
This is fast but only possible for 1:*-relations since linking-tables are mapped away.
Removes the readablity advantage of EF by using Ids after all
The navigation-properties of type EntityCollection<T> are not populated
Eager loading though the .Include()-methods, included for completeness (asking for lazy-loading)
Alledgedly joins everything included together and returns one giant flat result.
Have to decide up front which data to use
It there some way to get the simplicity of 2 with the performance of 3?
You could attach the parent object to your context and get the children when needed.
foreach (T parent in parents) {
_context.Attach(parent);
}
var children = parents.Select(p => p.Children);
Edit: for attaching multiple, just iterate.
I think finding a good answer is not possible or at least not worth the trouble. Instead a micro ORM like Dapper give the big benefit of removing the need to map between sql-columns and object-properties and does it without the need to create a model first. Also one simply writes the desired sql instead of understanding what linq to write to have it generated. IQueryable<T> will be missed though.